Composite structures typically include reinforcing fibers embedded in a resin matrix. Composite structures are designed to transmit loads along the length of the fibers. The resin matrix holds the fibers in position and may also transfer loads from fiber to fiber by passing the loads through the resin matrix. During the loading of a composite structure, the resin matrix is typically loaded in multiple directions. For example, when a load is placed on a composite structure, the resin matrix may be loaded along a direction parallel to the fibers and also along one or more directions transverse to the fibers. The resin matrix may fail when a certain combination of strains exceeds a predetermined value.
The strain invariant failure theory is a criteria that may be used to predict the combination of strains that will result in failure of the resin matrix in a composite structure. In the strain invariant failure theory, failure occurs when a first strain invariant J1 is exceeded. The first strain invariant J1 is the sum of the three principal strains (ε1+ε2+ε3) which are oriented orthogonally relative to one another.
When a standard test coupon is loaded in uniaxial tension, the principal strain ε1 increases in the tensile direction while the principal strains ε2+ε3 are negative or compressive as the coupon cross-section contracts due to Poisson's ratio. As a result of the ability of the coupon cross-section to contract as the coupon is stretched under the tension load, the principal strain ε1 may increase to a value significantly higher than if the coupon were unable to contract. In contrast, a material loaded in hydrostatic tension (i.e., ε1=ε2=ε3) will fail with a relatively low strain in the three principal directions.
When a load is placed on a composite structure, the reinforcing filaments typically constrain the resin matrix against contraction. The reduced capability of the matrix to contract limits the tensile strain capability of the matrix. As a result of the limit on the strain capability of the matrix, the performance of the composite structure may be limited. For example, when a composite structure is loaded in tension, the relatively low failure strain of the resin may result in failure of the resin (e.g., microcracking) prior to the fibers reaching their failure strain.
As can be seen, there exists a need in the art for a system and method for increasing the strain capability of the resin in a composite structure as a means to improve the performance of the composite structure.